Have a look at CGI::Enurl and CGI::Deurl or URI::Escape.

For example:

use strict;
use URI::Escape;

while (my $string = <DATA>) {
    chomp $string;
    if ($string eq uri_unescape($string)) {
        print "'$string' was not URI-encoded.\n";
    }
    else {
        print "'$string' was URI-encoded.\n";
    }
}
__DATA__
A%20horse%2C%20a%20horse!%20My%20kingdom%20for%20a%20horse!
A plain string
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